A.C. electric motors having a stator forming orthogonal magnetic paths

ABSTRACT

A stator of an A.C. electric motor includes a magnetic core comprising a magnetic ring member and two &#34;U&#34;-shaped magnetic members connected to the ring member. The two &#34;U&#34;-shaped magnetic members hold the ring member therebetween and are angularly spaced from one another around the central axis of the ring member. A primary winding is wound on one of the two &#34;U&#34;-shaped member and a secondary winding is wound on the other &#34;U&#34;-shaped member. The secondary winding is short-circuited by a capacitor. When A.C. voltage is applied to the primary winding, A.C. voltage is induced on the secondary winding at a certain value or more of the primary voltage. A rotary flux is formed in the ring member by magnetic fluxes induced by primary and secondary A.C. currents. Accordingly, a cage rotor or magnet rotor which is rotatably disposed in the ring member can be rotated.

This application is a continuation of application Ser. No. 207,084, filed 11/14/80.

BACKGROUND OF THE INVENTION

This invention relates to A.C. electric motors and, in particular, to improvements of stators of A.C. electric motors.

In known A.C. electric motors such as induction motors and synchronous motors, a stator must be formed with slots in its cylindrical surface confronting the rotary surface of a rotor with a magnetic gap therebetween, in which slots armature windings are disposed and electrically insulated. Because poor formation of slots and poor disposition of windings cause unevenness in rotation of the motor, much care and long time are disadvantageously taken to produce the stator.

Rotating speed of the known A.C. electric motor can be controlled by adjusting the applied A.C. voltage, but this means that the rotating speed changes by variations of the applied A.C. voltage.

SUMMARY OF THE INVENTION

Therefore, a main object of this invention is to provide an A.C. motor having a stator which is simple in construction and production.

Another object of this invention is to provide an A.C. motor having a stator wherein a rotary magnetic field is generated by a novel principle in order to achieve the above described object.

Still another object of this invention is to provide an A.C. motor the rotating speed of which is maintained constant even if the applied voltage is varied.

Yet another object of this invention is to provide an A.C. motor having little unevenness in rotation.

According to this invention, an A.C. electric motor comprises a stator including a magnetic body having a penetrated circular bore. First magnetic yoke means are connected to the magnetic body so that its opposite ends may be connected to one axial end surface of the magnetic body at the positions of the opposite end of a first diameter of the circular bore. Second magnetic yoke means are connected to the magnetic body so that its opposite ends may be connected to the other axial end surface of the magnetic body at the positions of the opposite ends of a second diameter angularly spaced from the first diameter. First winding means are wound on the first magnetic yoke means and have first electric terminals to which an A.C. voltage source should be connected. Second winding means are wound on the second magnetic yoke means and have second electric terminals. Capacitor means are connected to the second electric terminals. Rotor means are disposed in the circular bore of the magnetic body with its rotary shaft aligned with the center axis of the circular bore.

A cage rotor or magnet rotor may be used as the rotor means.

Further objects, features and other aspects of this invention will be understood from the following description of preferred embodiments of this invention with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a perspective view showing the structure of one preferred embodiment of the present invention;

FIGS. 2 and 3 are diagrams for explaining the principle of generation of a rotating magnetic field in the apparatus according to the present invention, FIG. 2 being a diagram showing the case where the input and output voltages are identical to each other, FIG. 3 being a diagram showing the case where the input and output voltages are different from each other;

FIG. 4 is a view showing the structure of one preferred embodiment of the electric motor according to the present invention, a perspective view being shown at (a), and an exploded perspective view being shown at (b);

FIG. 5 is a diagram showing the relations between the input voltage of the electric motor in FIG. 4 and the rotational speed, output voltage, input current and output current, respectively;

FIGS. 6(a) to 6(g) are diagrams showing an input voltage waveform (a), an input current waveform (b), an output voltage waveform (c), an output current waveform (d) and a Lissajous's figure representing a rotating magnetic field (d) for different input voltages with respect to the electric motor shown in FIG. 4;

FIG. 7 is a diagram showing the relations between the input voltage and the slip for various values of the capacitance of the resonant capacitor;

FIG. 8 is a diagram showing the relation between the input voltage and the starting torque;

FIG. 9 is a diagram showing the relations under a fixed torque condition between the input voltage and the output voltage, rotational speed, input current, output current and slip;

FIG. 10 is a diagram showing the slip-torque characteristics under a fixed input voltage condition;

FIGS. 11(a) to 11(c) are schematic views showing three other examples of the rotor;

FIG. 12 is a side view showing a structure of a magnet rotor used in the Embodiment 2;

FIGS. 13 and 14 are diagrams showing the relations between the input voltage and the input current, output current, output voltage and rotational speed, with respect to two different capacitance values for the resonant capacitor in the Embodiment 2 above;

FIG. 15 is a diagram showing the relation between the deflection angle between the respective magnetic yokes and the starting torque in the Embodiment 3;

FIG. 16 shows no-load characteristic curves of the electric motor in the Embodiment 4;

FIG. 17 is a diagram showing the relations between the capacitance of the resonant capacitor and the input voltage for three different ring members having different dimensions in the Embodiment 5;

FIG. 18 is a diagram showing the results of the no-load test and the lock test in the Embodiment 5;

FIG. 19 is a diagram showing the relation between the mechanical loss and the rotational speed;

FIG. 20 is a diagram showing the relation between the primary voltage and the starting torque;

FIG. 21 shows an equivalent circuit of the electric motor according to the present invention;

FIG. 22 is a diagram showing a gap magnetic flux during no-load operation;

FIG. 23 shows loading characteristic curves in the Embodiment 5; and

FIG. 24 is a diagram showing the relations between the gap length and the gap magnetic flux density.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

This invention is based on my following novel knowledge:

In a magnetic core including a magnetic ring member, a first "U"-shaped magnetic member connected to an axial end of the ring member, and a second "U"-shaped magnetic member connected to the other axial end of the ring member at an angular position different from the first magnetic member, a certain oscillation phenomenon is obserbed that, upon applying A.C. voltage to a primary winding wound on the first "U"-shaped magnetic member, A.C. voltage does not present on a secondary winding wound on the second "U"-shaped magnetic member and short-circuited by a capacitor at a lower primary voltage but suddenly presents at an increased primary voltage or more. The secondary voltage is observed to be different from the primary voltage by π/2 in the phase. The oscillation will be referred to as parametric oscillation. This means that a resultant flux of a magnetic flux induced by the primary current and another magnetic flux induced by the secondary current composed at the ring member rotates.

Referring now to FIG. 1, an A.C. electric motor according to one preferred embodiment of the present invention comprises a stator 1 including a ring-shaped member 11 made of magnetic material, and magnetic yokes 12 and 13 disposed on the axially opposite sides of the ring-shaped member 11. The magnetic yoke 12 is formed in a U-shape and its respective ends are jointed to one end surface of the ring-shaped member 11 at the positions of the opposite ends of one diameter of the member 11. The other magnetic yoke 13 is also formed in a U-shape and its respective ends are jointed to the other end surface of the ring-shaped member at the positions deflected in angle by 90° from the joining positions of the magnetic yoke 12. Around the magnetic yoke 12 is wound a winding 14 to be supplied with an A.C. voltage, while another winding 15 is wound around the magnetic yoke 13, and a capacitor 16 is connected across the winding 15.

In the bore of the ring-shaped member 11 is positioned a rotor 2, and the opposite ends of a rotary shaft 21 of the rotor 2 are rotatably supported by the magnetic yokes 12 and 13 via bearings (not shown).

Assuming now that an A.C. voltage is applied from an A.C. power supply S to the winding 14 by closing a power supply switch SW and a power supply voltage e₁ is increased, then parametric oscillation would arise abruptly at a certain value of the power supply voltage e₁, and consequently, a voltage e₂ that is 90° out of phase with respect to the input voltage e₁ and approximately equal in magnitude to the input voltage e₁, is induced in the other winding 15. At this moment, a rotating magnetic field is generated in the ring-shaped member 11, and thus the rotor 2 rotates.

In the followings, the principle of rotation of the rotor will be explained with respect to the case where a cage rotor is employed as the rotor 2.

With reference to FIG. 2, a magnetic flux Φ_(z) generated by the input voltage e₁ will vary along a time axis as shown at (a) in FIG. 2. On the other hand, a magnetic flux Φ_(y) generated by the voltage e₂ induced in the winding 15 will vary along a time axis as shown at (b) in FIG. 2. It is to be noted that at (a) and (b) are also shown the variations of the voltages e₁ and e₂, respectively, by dash-dot lines. On the other hand, since the magnetic yokes 12 and 13 are joined to the ring-shaped member 11 at angles from each other by 90°, the resultant magnetic flux Φ_(R) in the ring-shaped member formed from both magnetic fluxes Φ_(z) and Φ_(y) is as shown in FIG. 2(c), that is, the resultant magnetic flux Φ_(R) rotates in the anticlockwise direction as time elapses. In other words, a rotating magnetic field is generated in the ring-shaped member 11, and accordingly, the rotor 2 rotates with a slip S.

While the magnitude of the rotating magnetic field Φ_(R) is not varied with time as shown in FIG. 2(c) when the primary voltage e₁ is equal to the secondary voltage e₂, if the primary voltage e₁ is larger than the secondary voltage e₂, the magnitude Φ_(R) would change with time as shown in FIG. 3.

In general, the magnetic fluxes Φ_(y) and Φ_(z) produce a complex three-dimensional distribution within the ring-shaped member 11, and so, it is difficult to maintain the magnitude of the rotating magnetic field Φ strictly constant. However, according to results of experiments, it could be made approximately constant to obtain a substantially circular locus as shown in FIG. 2(c).

It is to be noted that if the input voltage e₁ is further increased, the parametric oscillation will cease at a certain voltage value, but the rotor 2 continues to rotate. In this case, the motor is operating as a single-phase induction motor excited only by the input winding 14.

Now description will be made of preferred embodiments of the A.C. electric motor according to the present invention.

EMBODIMENT 1

FIG. 4 is a schematic perspective view showing a structure of an electric motor used in the experiments, in which a Mn-Zn series ferrite ring was used as a ring member 11', commercially available cut cores (0.35 mm in thickness) made of Fe-Si series alloy were used as magnetic yokes 12' and 13', and 500 turns of wire were wound around the respective yokes as windings 14' and 15', respectively. A cage rotor for use in an induction motor was used as a rotor 2'. The magnetic yokes 12' and 13' made of cut cores were fixedly placed on bakelite plates 3 and 3', respectively, the ring member 11' was placed on one cut core 13', the rotor 2' was disposed within the ring member, the cut core 12' was placed on the ring member 11' at an angle of 90° with respect to the cut core 13', connecting rods 4 were penetrated through the both bakelite plates, nuts 5 were threadedly engaged with the respective connecting rods 4 to fix the entire assembly, and thus the electric motor was built. With regard to the support for the rotary shaft of the rotor 2', fixing plates 6 and 6' containing bearings therein were respectively disposed on the inner surfaces of the cut cores 12' and 13', and the opposite ends of the rotary shaft 21' of the rotor 2' were made to be supported by the respective bearings.

In addition, on the bakelite plates 3 and 3' are provided terminal fittings, 7, 7' and 8, 8' connected to the windings 14' and 15', respectively.

Between the terminals 8 and 8' to which the winding 15' was connected, was connected a capacitor having a capacitance of 500 μF was connected as a resonant capacitor (16 in FIG. 1), an A.C. power supply of 50 Hz in frequency (also the same in the followings) was connected to the terminals 7 and 7' to which the winding 14' was connected, and a current flowing through the winding 14' (hereinafter called input current) I₁, a current flowing through the winding 15' (this being an induced current and hence hereinafter called output current) I₂, a voltage generated across the winding 15' (this being an induced voltage and hence hereinafter called output voltage) E₂ and a rotational speed N (r.p.m.) of the rotor 2' were measured while the power supply voltage E₁ is varied. The results are shown in FIG. 5.

As shown in FIG. 5, when the input voltage E₁ reaches 30 V, parametric oscillation arises, then the output voltage E₂ becomes 34 V, and at the same time the rotor 2' begins to rotate at 1300 r.p.m. As the input voltage E₁ is further raised, the output voltage E₂ decreases, but the rotational speed N increases. When the input voltage E₁ becomes about 50 V, the parametric oscillation ceases. However, it is seen that since the excitation through the input winding 14' becomes intense, the motor operates as a single-phase induction motor and the rotation continues.

Next, the electric motor shown in FIG. 4 was used, a capacitor having a capacitance of 66 μF was employed as the resonant capacitor, and waveforms of the input voltage, input current, output voltage and output current as well as a Lissajous's figure representing the rotating magnetic field generated in the ring member 11' were observed. The waveforms and Lissajous's figures observed at the input voltages of 10 V, 20 V, 30 V, 40 V, 50 V, 60 V and 70 V are illustrated at (A), (B), (C), (D), (E), (F) and (G), respectively, in FIG. 6. In the respective sections (A) to (G), (a) represents the input voltage waveform, (b) represents the input current waveform, (c) represents the output voltage waveform, (d) represents the output current waveform and (e) represents the Lissajous's figure.

From FIG. 6, it can be seen that when the input voltage is increased to 40 V or above, a circular rotating magnetic field is generated.

It is to be noted that in the electric motor used in these experiments, because of the fact that the bearings for the rotor 2' were not perfect and accompanied by a large frictional loss, virtually a heavy load was applied to the motor, and consequently, the rotational speed of the rotor 2' was 1350 r.p.m. at the maximum which corresponded to a slip S of about 55%. However, the slip S could be improved to about 10% by improving the bearings and varying the resonant capacitor (16 in FIG. 1). The mode of improvement is illustrated in FIG. 7.

Next, the A.C. electric motor shown in FIG. 4 was used, a capacitor having a capacitance of 60 μF was used as the resonant capacitor, and the relation between the input voltage E₁ and a starting torque Ts as well as the relations under a fixed load condition between the input voltage E₁ and the output voltage E₂, rotational speed N, input and output currents I₁ and I₂ and slip S, respectively, were measured. The results are illustrated in FIGS. 8 and 9, respectively. It is seen from FIG. 9 that there exists a region where the rotational speed remains constant even if the power supply voltage E₁ should somewhat vary, provided that the load is kept constant. In addition, FIG. 10 shows slip-torque characteristics when the input voltage is 45 V.

While a cage rotor was used as the rotor in the above-described embodiment, magnet rotors as shown in FIG. 11 which are used in a synchronous motor, could be used as the rotor. In this case, the rotor could be either a dipole rotor as shown at (a) or (b) or a multi-pole rotor as shown at (c).

Now description will be made on other preferred embodiments employing a magnet rotor.

EMBODIMENT 2

In the stator magnetic paths having parametric oscillation characteristics consisting of orthogonally crossed cores as used in Embodiment 1 (shown in FIG. 4), was assembled a magnet rotor 2" as shown in FIG. 12, and the assembly was operated. Regarding the material of the rotor then used, an Al-Ni-Co system magnet (TMK-4 material manufactured by Tohoku Metal Industries, Ltd.) having a diameter D=44.5 mm and a length l=25 mm was used, and the magnetization was dipole magnetization.

Then the no load characteristics, that is, the measured values of the input current I₁, output voltage E₂, output current I₂ and rotational speed N as functions of the input voltage E₁ are illustrated in FIGS. 13 and 14. FIG. 13 shows the characteristics when the resonant capacitor connected across the secondary winding has a capacitance C=60 μF, and FIG. 14 shows the characteristics when the resonant capacitor has a capacitance C=68 μF. From these results it is seen that the motor is operating as a synchronous motor.

In this case also, the rotor does not rotate at all in the nonoscillation region, but when it enters the parametric oscillation region, that is, the circular rotating magnetic field region, it suddenly starts to rotate, I₂ and N take extremely stable values, and as the input voltage is increased when the parametric oscillation ceases and it enters an elliptic rotating magnetic field region, it becomes a general synchronous motor characteristics region and thus I₂ decreases. However, if an extremely favorable capacitance value is selected for the resonant capacitor, then parametric oscillation can be generated over a wide range of the input voltage E₁, and one example of such case is the case shown in FIG. 14.

Although it is known that as the input voltage E₁ is successively increased, once the rotor starts to rotate, it sustains rotation with the synchronous motor characteristics even if the secondary resonant capacitor is disconnected, it is also known that if the resonant capacitor is disconnected when the rotor is stationary, then the rotor cannot rotate at all to whatever extent the input voltage E₁ may be increased. However, at this moment, if a little torque is applied to the shaft of the rotor, then it suddenly starts to rotate. As a result, the resonant capacitor has the role of a starter.

While description was made in connection to the case where the magnetic yokes 12 and 13 are offset in angle by 90° from each other about the axis of the ring-shaped member 11, in the above-described respective embodiments, even if this offset in angle is not 90°, the apparatus must operate equally as an electric motor as long as an offset in angle exists, because the magnetic fluxes passing through the respective magnetic yokes would interlink in the ring-shaped member 11. However, it is considered that in the case where the offset in angle is not 90°, the motor characteristics would be somewhat degraded as compared to the case where it is 90°. On the basis of the above-mentioned consideration, the following experiments were conducted.

EMBODIMENT 3

The A.C. electric motor used in Embodiment 1 was also used in this embodiment, the offset in angle θ between the first magnetic yoke 12' on the primary and the second magnetic yoke 13' on the secondary was varied in the range of 0° to 180°, and experiments were conducted in the region of parametric oscillation characteristics at the respective angles. The starting torque characteristics observed in these experiments are shown in FIG. 15. From this figure, it is seen that the maximum torque is obtained when the offset in angle θ is 90°, the torque decreases as it deviates from 90°, but so long as an offset in angle exists the motor can operate as an A.C. electric motor having parametric characteristics.

EMBODIMENT 4

In the structure of the electric motor shown in FIG. 4, a silicon steel plate (0.35 mm in thickness) wound in a ring shape was used as the ring-shaped member 11', commercially available cut cores (0.35 mm in thickness) made of Fe-Si series alloy was used as the magnetic yokes 12' and 13', and 500 turns of wire were used as the windings 14' and 15' around the respective yokes. A cage rotor for use in a single-phase induction motor was employed as the rotor 2'.

Experiments were conducted by making use of the above-described electric motor. FIG. 16 shows no-load characteristic curves of the experimental motor. In order to obtain an oscillation start voltage of about 100 V, a capacitor having a capacitance C=20 μF was used as the resonant capacitor (16 in FIG. 1).

With reference to FIG. 16, parametric oscillation starts at the input voltage E₁ =96.7 V, then the primary current I₁ and the primary input power W₁ increase abruptly, and thereafter as the primary voltage E₁ increases, they also rise. On the side of the secondary winding also, at the same time as the start of oscillation the secondary current I₂ and the secondary voltage E₂ increase abruptly, and thereafter as the primary voltage E₁ increases, they decrease. In addition, the secondary power W₂ presents very small values. The rotor rotates simultaneously with the start of oscillation, and the rotational speed is N=2800 r.p.m.

EMBODIMENT 5

Next, for the purpose of investigating the influence of the magnetic ring member 11 serving as a common magnetic path upon the characteristics of the motor, in the motor used in Embodiment 4 three kinds of ring members as identified in the following table were prepared and interchangeably employed. For simplicity of consideration, the lengths and inner diameters of the three members were chosen equal to each other and only the outer diameters were varied.

    ______________________________________                                         Ring Members   (I)         (II)   (III)                                        ______________________________________                                         Inner Diameter (mm)                                                                           47.7        47.7   47.7                                         Outer Diameter (mm)                                                                           67.7        64.7   60.9                                         Length (mm)    30.0        30.0   30.0                                         ______________________________________                                    

FIG. 17 shows the results of measurement for the relations between the input voltage E₁ upon start of oscillation and the capacitance C of the resonant capacitor as measured while the above-described common magnetic paths are interchangeably employed and the capacitance is varied. Referring to this figure, the input voltage E₁ upon start of oscillation rises linearly with the increase of the capacitance C. From this figure it is seen that when it is desired to select the input voltage E₁ upon start of oscillation at 100 V, the capacitance may be selected at C=20 μF in the case of the common magnetic path (I) or at C=36 μF in the case of the common magnetic path (II).

EMBODIMENT 6

Next, in order to estimate the characteristics by means of an equivalent circuit, a no-load test and a lock test were carried out with respect to the motor employing the ring member (I) as referred to in Embodiment 5. The test results are illustrated in FIG. 18.

In the no-load test, in order to eliminate the influence of mechanical loss, the test motor was driven by another electric motor, and measurements were made at the rotational speed N=3000 r.p.m. The results of the no-load test are represented by adding a suffix "0". That is, I₁₀ and W₁₀ represent a current and power, respectively, on the primary input side, and I₂₀, E₂₀ and W₂₀ represent a current, voltage and power, respectively, on the secondary side where parametric oscillation is generated. Reviewing FIG. 18, the secondary power W₂₀ presents very small values.

The results of the lock test are also illustrated in the same figure. This was done for the sake of convenience, taking into consideration the convenience that since the impedance voltage of the motor was substantially equal to the no-load voltage the results of the lock test can be easily compared with the results of the no-load test. The results of the lock test are represented by adding a suffix "s". With reference to FIG. 18, it is seen that with regard to all the primary input power W_(1s), primary current I_(1s), the secondary voltage E_(2s) and secondary current I_(2s), the results of the lock test present the same tendency as the results of the no-load test. In addition, the secondary power W_(2s) has very small values.

The resistances r₁ and r₂ of the windings 14' and 15', respectively, are as follows:

    r.sub.1 =2.9Ω

    r.sub.2 =3.5Ω

(the temperature upon measurement being 25° C.)

FIG. 19 shows the results of measurement for the relation between the mechanical loss and the rotational speed N (r.p.m.). Referring to this figure, although some fluctuations are acknowledged in the measured values, the mechanical loss Wm increases approximately in proportion to the square of the rotational speed, and the mechanical loss Wm=1 W is observed at the rotational speed N=2500 r.p.m.

FIG. 20 shows the results of measurement of the relation between the primary voltage E₁ and the starting torque T_(st). The starting torque T_(st) takes the maximum value at the primary voltage E₁ =120 V, and the maximum value is T_(st) =78 g-cm.

As an equivalent circuit, an equivalent T-network for a symmetric two-phase induction machine as shown in FIG. 21 was employed, upon determination of the circuit constants the method was employed in which the resistance r_(r) and the reactance x_(r) of the rotor are separated by determining the ratio of the reactance x_(r) to the resistance r_(r), that is, k=x_(r) /r_(r), and moreover, investigation was also made on the method in which the leakage reactance of the stator is actually measured and thereby the circuit constants are determined.

FIG. 22 shows the results of measurement of a gap magnetic flux during no-load operation. The magnetic flux was measured by means of eight search coils disposed at equal intervals along the inside of the common magnetic path. In order to satisfy the condition of the two-phase induction motor, magnetic excitation was made by applying respective voltages which are equal in magnitude but different in phase by 90° from separate power supplies. With reference to FIG. 22, the gap magnetic flux is approximately of circular shape, and hence it is seen that the magnetic flux is substantially uniformly distributed within the gap. When the primary and secondary voltages are E₁ =E₂ =100 V, a magnetic flux density of B=600 gausses was obtained.

In FIG. 23 are shown loading characteristic curves measured at a fixed primary voltage E₁ =96.7 V=constant in the case where the capacitance of the capacitor C=20 μF is selected. In this figure, solid lines represent measured values, whereas dash lines represent calculated values which were estimated from the equivalent circuit in FIG. 21. With reference to this figure, it is seen that with respect to the primary current I₁ and the primary input power W₁ the measured values fairly well coincide with the calculated values, but considerable errors exist between the measured values and the calculated values of the torque T. Consequently, the errors are also large with respect to the output P_(m) and the efficiency η. However, since they are relatively coincident in their general tendencies, it is believed that the equivalent circuit shown in FIG. 21 is available provided that the circuit conditions are preset so as to fulfil the condition of E₁ =E₂.

In addition, it is observed that the torque curve takes the maximum value upon starting and it decreases as the slip S is reduced, and this is considered to be due to the fact that the resistance of the rotor as used has a very large value.

Now, investigation will be made on the values of the output power and efficiency. With reference to FIG. 23, they present very small values such as P_(m) =0.7 W and η=about 7%, and this is due to the fact that the gap magnetic flux is small. More particularly, in the experimental motor, for the purpose of measuring the gap magnetic flux, the gap length is selected at a relatively large value of g=1.6 mm. Consequently, the gap magnetic flux density takes a very small value of B=600 Gausses (I_(f) =0.7A) as shown in FIG. 24. If the gap magnetic flux density is increased up to the value employed in the conventional motors such as several Kilo-Gausses by reducing the gap length and increasing the excitation ampere-turns, then since the torque is proportional to the square of the gap magnetic flux, it is possible to widely improve both the output power and the efficienty.

The present invention has been described above in connection to its preferred embodiments. Now the characteristic advantage of the present invention will be enumerated in the followings:

(1) When the rotor is rotating under the parametric oscillation, the motor posseses a filtering function similarly to the case where a parametric transformer is used, and hence there is no need to use a special power supply.

(2) The windings can be separately wound around the first and second magnetic yokes, respectively, and one can spare the complexed process of providing slots on the gap sides of both the stator and the rotor and mounting conductors in these slots as insulated therefrom as is the case with the conventional electric motors. Hence the assembling work is very simple. In addition, since unevenness of the magnetic field is not produced, there exists little unevenness in rotation.

(3) Upon parametric oscillation, if the input voltage and the output voltage are made equal, an approximately circular rotating magnetic field can be obtained. Whereas, in the single-phase induction motor it is difficult to obtain a circular rotating magnetic field by making use of an inductor or a capacitor, and so, the motor according to the present invention is superior to the prior art motor.

(4) Even if the parametric oscillation should cease, the motor would continue to rotate as an induction motor or a synchronous motor.

(5) Provided that the load is kept constant, there exists an operating region where the rotational speed can be maintained constant even if the power supply voltage should somewhat vary.

(6) Not only a motor operated by a commercial power supply, but also an electric motor for operating at a high frequency such as 1 KHz can be realized.

(7) Regardless of whether the motor is large or small in size, the manufacture of the motor is easy. 

What is claimed is:
 1. An A.C. electric motor comprising:a stator including a complete ring of magnetic material having a circular bore extending axially therethrough, said ring having opposite end surfaces, first magnetic yoke means having one end connected to one end surface of said magnetic ring at opposite end positions of a first diameter of said circular bore, second magnetic yoke means having one end connected to the other end surface of said magnetic ring at opposite end positions of a second diameter angularly spaced from said first diameter, first winding means wound on said first magnetic yoke means and having first electric terminals to which an A.C. voltage source is adapted for being connected, and second winding means wound on said second magnetic yoke means and having second electric terminals; capacitor means connected to said second electric terminals; and rotor means disposed in said circular bore of said magnetic ring and including a rotary shaft aligned with the axis of said circular bore.
 2. An A.C. electric motor as claimed in claim 1, wherein each of said first and second magnetic yoke means is a magnetic member of U-shaped section, opposite ends of said rotary shaft being rotatably supported in base portions of the "U" of said magnetic members.
 3. An A.C. electric motor as claimed in claim 2, wherein said first winding means is mounted on a leg portion of the "U" of said first U-shaped magnetic member, and wherein said second winding means is mounted on a leg portion of the "U" of said second U-shaped magnetic member.
 4. An A.C. electric motor as claimed in claim 3, wherein each of said first and second winding means has 500 turns of wire.
 5. An A.C. electric motor as claimed in claim 4, wherein said capacitor means has a capacitance of 500 μF.
 6. An A.C. electric motor as claimed in claim 4, wherein the capacitance of said capacitor means is in a range of 20-70 μF.
 7. An A.C. electric motor as claimed in claim 6, wherein the capacitance of said capacitor means is 68 μF.
 8. An A.C. electric motor as claimed in claim 2, wherein said rotor means comprises a cage rotor.
 9. An A.C. electric motor as claimed in claim 2, wherein said rotor means comprises a magnet rotor is used as said rotor means.
 10. An A.C. electric motor as claimed in claim 2, wherein said magnetic ring is of Mn-Zn system ferrite, and each of said first and second "U"-shaped magnetic members is a cut core of Fe-Si system alloy.
 11. An A.C. electric motor as claimed in claim 9, wherein said magnet rotor is made of Al-Ni-Co system magnet materials.
 12. An A.C. electric motor as claimed in claim 1 wherein said opposite end surfaces extend parallel to one another and perpendicular to the axis of said central bore.
 13. An A.C. electric motor as claimed in claim 1 wherein said complete ring of magnetic material surrounds said rotor in entirety to form a continuous gap therewith.
 14. An A.C. electric motor comprising a continuous ring of magnetic material having a circular bore extending axially therethrough, said ring having opposite end surfaces, first magnetic yoke means connected to one of said end surfaces of said ring, second magnetic yoke means connected to the other of said end surfaces of said ring, said first and second magnetic yoke means being angularly offset from one another, first winding means on said first magnetic yoke means for being energized by A.C. voltage, second winding means on said second magnetic yoke means, capacitor means connected to said second winding means, and rotor means in said circular bore forming a continuous, unbroken gap with said magnetic ring.
 15. An A.C. electric motor as claimed in claim 14 wherein said rotor means is coaxially mounted in said bore.
 16. An A.C. electric motor as claimed in claim 15 wherein each of said yoke means comprises a magnetic member of U-shaped section including a pair of arms connected to said magnetic ring and a base connecting said arms together.
 17. An A.C. electric motor as claimed in claim 16 wherein said rotor means comprises a rotary shaft rotatably supported in the bases of the magnetic members of said first and second yoke means.
 18. An A.C. electric motor as claimed in claim 17 wherein said magnetic members are angularly offset by about 90°.
 19. An A.C. electric motor as claimed in claim 17 wherein said first and second winding means are respectively wound on one of the legs of the magnetic members of U-shaped section.
 20. An A.C. electric motor as claimed in claim 16 wherein said opposite end surfaces extend parallel to one another and perpendicular to the axis of said central bore.
 21. An A.C. electric motor as claimed in claim 1 wherein said first and second magnetic yoke means respectively form with said ring closed magnetic circuits having a common magnetic path through said ring, said ring forming a continuous, unbroken gap with said rotor means to provide a constant rotating magnetic field and uniform and even rotation of said rotor means.
 22. An A.C. electric motor as claimed in claim 14 wherein said first and second magentic yoke means respectively form with said ring closed magnetic circuits having a common magnetic path through said ring, said continuous, unbroken gap between the ring and rotor means providing a constant rotating field and uniform and even rotation of said rotor means. 